The most recent of these that I've come across is http://printablecoldsores.blogspot.com/ - offering downloadable images that you print onto sticker transparencies and put onto billboards and other advertising posters etc to highlight the unreality of corporate images of beauty.
Geek Graffiti - the creator of these - makes the point that defacing advertising etc is illegal and encourages people to put the stickers on, photograph them and remove them - which seems like a reasonable enough degree of legalistic self preservation. (And obviously I would neeeever encourage people to knowingly break the law either and leave them there.)
Almost makes me sad that Canberra is a city with a ban on public billboards.
Great work Geek Graffiti.
